An investigation of the developmental expression of calretinin (CR) immunoactivity and mRNA expression carried out in the developing mouse forebrain revealed a prominent structure between the thalamus and hypothalamus/preoptic area named thalamic eminence in the older literature. This little researched area is present in the fetal mammalian diencephalon and adult lower vertebrates. Calretinin-positive staining was first observed in the mouse thalamic eminence at embryonic day 11. In situ hybridization histochemistry confirmed the presence of calretinin mRNA in the thalamic eminence. By postnatal day 0 the thalamic eminence was no longer discernable. The function of the thalamic eminence is unknown. We have suggested that it may act as an organizing center for the diencephalon. The fact that CR immunoreactivity is intensively expressed in the fetal thalamic eminence is suggestive that CR expression during the fetal period is related to the function of the thalamic eminence during development. A series of experiments were undertaken in order to determine whether the thalamic eminence cells were undifferentiated. The brains of E-14 day rat embryos were fixed in formalin and brain sections were cut in a cryostat. Colocalization studies of calretinin and nestin (a marker for undifferentiated cells) demonstrated that calretinin and nestin were not colocalized. Furthermore no localization of calretinin with PCNA (a marker for mitotic cells) was observed. A study of the colocalization of BrdU (another marker for undifferentiated cells in mitosis) also revealed no colocalization with calretinin. Calretinin did, however colocalize with TuJ1 (a marker for migrating neurons). We suggest that calretinin cells migrate from the thalamic eminence to other regions of the brain (hypothalamus, thalamus, amygdala, cerebral cortex, and olfactory bulb) rather than disappearance by programmed cell death.